java.util.ConcurrentModificationException when removing elements from a hashmap

Question

I am learning about HashMap class and wrote this simple program. this code works good for adding elements to the hashmap and while removing elements from the hashmap , I am encountering java.util.ConcurrentModificationException for example here is a copy of my terminal,

[ravi@doom test]$ java TestHashMap 
.....MENU.....
1. Add
2. remove key
3. remove value
4. display
7. Exit
Your choice :1

 Key : A

 Value : 1
Key/Value : (A,1) added to storage.
.....MENU.....
1. Add
2. remove key
3. remove value
4. display
7. Exit
Your choice :1

 Key : B

 Value : 2
Key/Value : (B,2) added to storage.
.....MENU.....
1. Add
2. remove key
3. remove value
4. display
7. Exit
Your choice :1

 Key : C

 Value : 3
Key/Value : (C,3) added to storage.
.....MENU.....
1. Add
2. remove key
3. remove value
4. display
7. Exit
Your choice :1

 Key : D

 Value : 4
Key/Value : (D,4) added to storage.
.....MENU.....
1. Add
2. remove key
3. remove value
4. display
7. Exit
Your choice :4
( D , 4 );
( A , 1 );
( B , 2 );
( C , 3 );
.....MENU.....
1. Add
2. remove key
3. remove value
4. display
7. Exit
Your choice :2
Key to REMOVE : 
D
Pair (D,4) Removed.
.....MENU.....
1. Add
2. remove key
3. remove value
4. display
7. Exit
Your choice :4
( A , 1 );
( B , 2 );
( C , 3 );
.....MENU.....
1. Add
2. remove key
3. remove value
4. display
7. Exit
Your choice :3
Enter Value to remove : 2
Key : B Removed.
Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.HashMap$HashIterator.nextEntry(HashMap.java:922)
    at java.util.HashMap$EntryIterator.next(HashMap.java:962)
    at java.util.HashMap$EntryIterator.next(HashMap.java:960)
    at TestHashMap.start(TestHashMap.java:60)
    at TestHashMap.main(TestHashMap.java:87)
ABRT problem creation: 'success'

code:

import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

class TestHashMap
{
    private Map<String,Integer> map;
    public TestHashMap()
    {
        map = new HashMap<String,Integer>();
    }
    public void displayMenu()
    {
        System.out.println(".....MENU.....");
        System.out.println("1. Add");
        System.out.println("2. remove key");
        System.out.println("3. remove value");
        System.out.println("4. display");
        System.out.println("7. Exit");
        System.out.print("Your choice :");
    }
    public void start()
    {
        Scanner input = new Scanner(System.in);
        int menuChoice,value;
        String key;
        while(true)
        {
            displayMenu();
            menuChoice = input.nextInt();
            switch(menuChoice)
            {
                case 1:
                    System.out.print("\n Key : ");
                    input.nextLine();
                    key = input.nextLine();
                    System.out.print("\n Value : ");
                    value = input.nextInt();
                    map.put(key,new Integer(value));
                    System.out.println("Key/Value : ("+key+","+value+") added to storage.");
                break;
                case 2:
                    System.out.println("Key to REMOVE : ");
                    input.nextLine();
                    key = input.nextLine();
                    Integer v = map.get(key);
                    if(v == null)
                        System.out.println("No value exists for key "+key);
                    else
                    {
                        map.remove(key);
                        System.out.println("Pair ("+key+","+v.intValue()+") Removed.");
                    }
                    break;
                case 3:
                    System.out.print("Enter Value to remove : ");
                    value = input.nextInt();
                    if(map.containsValue(new Integer(value)))
                    {
                        for(Map.Entry<String,Integer> entry : map.entrySet() )
                        {
                            if(entry.getValue().intValue() == value)
                            {
                                System.out.println("Key : "+entry.getKey()+" Removed.");
                                map.remove(entry.getKey());
                            }
                        }
                    }
                break;
                case 4:
                    for(Map.Entry<String,Integer> entry : map.entrySet() )
                    {
                        System.out.println("( "+entry.getKey()+" , "+entry.getValue()+" );");
                    }
                break;
                case 7:
                    input.close();
                    System.exit(0);
                default:
                    System.out.println("Invalid Choice !");
            }
        }
    }
    public static void main(String args[])
    {
        TestHashMap thm = new TestHashMap();
        thm.start();
    }
}

UPDATE: working code

thanks to both (rgettman,Nathan Hughes) of you.

import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;
import java.util.Iterator;

class TestHashMap
{
    private Map<String,Integer> map;
    public TestHashMap()
    {
        map = new HashMap<String,Integer>();
    }
    public void displayMenu()
    {
        System.out.println(".....MENU.....");
        System.out.println("1. Add");
        System.out.println("2. remove key");
        System.out.println("3. remove value");
        System.out.println("4. display");
        System.out.println("7. Exit");
        System.out.print("Your choice :");
    }
    public void start()
    {
        Scanner input = new Scanner(System.in);
        int menuChoice,value;
        String key;
        while(true)
        {
            displayMenu();
            menuChoice = input.nextInt();
            switch(menuChoice)
            {
                case 1:
                    System.out.print("\n Key : ");
                    input.nextLine();
                    key = input.nextLine();
                    System.out.print("\n Value : ");
                    value = input.nextInt();
                    map.put(key,new Integer(value));
                    System.out.println("Key/Value : ("+key+","+value+") added to storage.");
                break;
                case 2:
                    System.out.println("Key to REMOVE : ");
                    input.nextLine();
                    key = input.nextLine();
                    Integer v = map.get(key);
                    if(v == null)
                        System.out.println("No value exists for key "+key);
                    else
                    {
                        map.remove(key);
                        System.out.println("Pair ("+key+","+v.intValue()+") Removed.");
                    }
                    break;
                case 3:
                    System.out.print("Enter Value to remove : ");
                    value = input.nextInt();
                    if(map.containsValue(new Integer(value)))
                    {
                        for (Iterator<Map.Entry<String, Integer>> it = map.entrySet().iterator();it.hasNext();) 
                        {
                            Map.Entry<String,Integer> x = it.next();
                            if(x.getValue().intValue() == value)
                            {
                                key = x.getKey();
                                it.remove();
                                System.out.println("Key : "+key+" Removed.");
                            }
                        }
                    }
                break;
                case 4:
                    for(Map.Entry<String,Integer> entry : map.entrySet() )
                    {
                        System.out.println("( "+entry.getKey()+" , "+entry.getValue()+" );");
                    }
                break;
                case 7:
                    input.close();
                    System.exit(0);
                default:
                    System.out.println("Invalid Choice !");
            }
        }
    }
    public static void main(String args[])
    {
        TestHashMap thm = new TestHashMap();
        thm.start();
    }
}

The problem is you are mutating the map in the same time that is getting an deleting. Put the system.out.println before the remove. — StackFlowed, Oct 21 '14 at 19:00
possible duplicate of [ConcurrentModification Exception with Map and Hashtable](http://stackoverflow.com/questions/13453217/concurrentmodification-exception-with-map-and-hashtable) — ᴇʟᴇvᴀтᴇ, Oct 21 '14 at 19:16

score 13 · Answer 1 · answered Oct 21 '14 at 19:01

You are calling remove while you're iterating over the Map. This line, the enhanced for loop, runs an Iterator implicitly:

for(Map.Entry<String,Integer> entry : map.entrySet() )

When an Iterator detects that its collection is modified, it throws a ConcurrentModificationException. However, you can call remove() on the Iterator itself without that exception being thrown. Use an Iterator explicitly:

Iterator<Map.Entry<String, Integer>> itr = map.entrySet().iterator();
while(itr.hasNext())
{
   Map.Entry<String, Integer> entry = itr.next();
   if(entry.getValue().intValue() == 2)
   {
      System.out.println("Key : "+entry.getKey()+" Removed.");
      itr.remove();  // Call Iterator's remove method.
   }
}

score 12 · Accepted Answer · answered Oct 21 '14 at 19:01

12

Your for-loop gets the map.entrySet and uses the iterator on it to work through the map entries (this version of the for-loop requires an Iterable, it gets the iterator from the Iterable). When you are using an iterator on a map, but remove things from the map without using that iterator, you get the ConcurrentModificationException. That is the map telling the iterator that it's out of date.

You can write a for loop using the iterator explicitly, like this:

for (Iterator<Map.Entry<String, Integer> it = map.entrySet().iterator();
it.hasNext();) {

and use the iterator's remove method when you need to delete an entry.

answered Oct 21 '14 at 19:01

Nathan Hughes

94,330
19
181
276

I prefer the version that uses `while(it.hasNext())`, as for loops are normally used for loop where the number of iteration are known in advance. Well I've been told so years ago at uni. – Julien Sep 02 '15 at 13:24
3

@Julien: sure, pretty much the same thing; only difference is the for loop keeps the scope of the iterator variable local to the for-block. – Nathan Hughes Sep 02 '15 at 13:50

score 2 · Answer 3 · answered Oct 21 '14 at 19:27

You cannot remove an element from a map which you are currently iterating over. You could define an iterator, or you could make a few simple modifications to your code within the block for case 3.

case 3:
    System.out.print("Enter Value to remove : ");
    value = input.nextInt();
    if(map.containsValue(new Integer(value)))
    {
        Map.Entry<String,Integer> foo = null;
        for(Map.Entry<String,Integer> entry : map.entrySet() )
            if(entry.getValue().intValue() == value)
                foo = entry;

        System.out.println("Key : "+foo.getKey()+" Removed.");
        map.remove(foo.getKey());
    }
    break;

score 1 · Answer 4 · answered Aug 23 '15 at 20:52

Just Use Iterator class to remove the Iterating Element as

for (Iterator<Map.Entry<String, Integer> it = map.entrySet().iterator();
while(it.hasNext()) {
  Map.Entry<String, Integer> e= itr.next();
  key = e.getKey();
  Value = e.getValue();

  //Your Other Code Here

   it.remove();                    //It removes the current Itertaion from Map


}

score 0 · Answer 5 · answered Dec 07 '21 at 08:46

0

It is possible to use removeIf of the Collections class.

map.keySet().removeIf(key -> key.equals("someThing"));

answered Dec 07 '21 at 08:46

SRF

587
5
16

java.util.ConcurrentModificationException when removing elements from a hashmap

5 Answers5

Linked